Hydroelectric turbine installations in which the turbine comprises several runner blades having an adjustable pitch are widely used. In these turbines, each runner blade (often simply called a "blade"), is pivotally connected to the hub and rotatable about an axis extending in a direction generally perpendicular to the hub. The rotation of each blade about its axis permits the turbine operator to seek optimum efficiency of the hydroelectric installation under the entire range of operating conditions of the turbine. In the hydroelectric industry, turbines wish adjustable pitch blades are commonly referred to as "Kaplan turbines". Kaplan turbines are also typically provided with adjustable wicket gates designed to regulate the flow of water admitted to the turbine.
It is well recognized that hydroelectric power generation is generally socially more desirable than its counterparts which obtain energy from the combustion of fossil fuel or the fission or fusion of atoms. It is also widely accepted that Kaplan turbines materially improve the efficiency of hydroelectric installations. However, it is increasingly being suspected that certain Kaplan installations have various detrimental impacts on the environment, particularly on the fish population which is present in the water flowing through the turbine.
One of these potentially adverse impacts results from the very features of Kaplan turbines that increase the efficiency of hydroelectric installations using these turbines. Specifically, in a Kaplan turbine having its main axis generally vertical, the pitch of the blades is adjustable from maximum blade opening or pitch, i.e., when the face of each blade is almost parallel to the main turbine axis, to minimum blade opening or pitch in which case the face of each blade is placed in a flatter position (i.e., generally perpendicular to the flow of water fed to the turbine). In Kaplan turbines having a non-fully spherical upstream hub, when the blade is at low pitch a gap is created between the hub and the leading edge of the blade (i.e., upstream of the axis of rotation of the blade relative to the hub), as illustrated for example in FIG. 26.
Various studies have shown that these gaps have several detrimental effects. First, a gap between the hub and the blade creates cavitation due to water leakage occurring through the gap. Significantly, the gap (particularly upstream of the blade rotational axis) is also a trap for fish which are present in the water flowing through the turbine. It is believed that fish flowing into the gap have a significantly greater chance of being injured or killed than fish flowing through other regions of the turbine.
Recent efforts have therefore been undertaken to address the apparent propensity of Kaplan turbines to injure fish. For example, systems have been designed to divert fish away from Kaplan turbines. These systems include screens to keep fish out of the turbine, or structures designed to divert fish away from the turbine. It can be readily appreciated, however, that these prior art structures have several shortcomings. First, systems of the type necessitating separate structures restrict the amount of water flowing through the turbine thereby reducing the efficiency of the turbine and causing energy losses. Second, it has been found that these systems are not fully effective to divert the entire fish population away from the turbine. In addition, screens turn into nets in which the fish are caught; the screens must therefore be rotated to discharge the fish caught in the screens in regions of the water not flowing through the turbine. Finally, as can be readily appreciated, these additional structures which, in addition to not being entirely satisfactory, materially increase the cost of hydroelectric installations using Kaplan turbines.
Various attempts have also been made to increase the efficiency of adjustable pitch propellers and turbines by reducing the gap formed in these mechanisms. For example, U.S. Pat. No. 2,498,072 issued Feb. 21, 1950 to Dean discloses an aircraft propeller in which the pitch of the blades is adjustable. To reduce air turbulence and drag in the region of the gap formed at the base of the blade, a seal made of molded rubber is attached to the hub embracing the blade airfoil. Still another example of an approach used to improve the operating characteristics of air propellers is illustrated in U.S. Pat. No. 2,378,958 issued on Jun. 26, 1945 to Troller. In Troller, to minimize noise and air turbulence, the rim of the propeller having a cylindrical outer surface is provided with a recess to receive the base portion of the blace. The close tolerance between the base of the blade and the recess generally improves the operation of the propeller. The inventors of the present invention are also aware of the use in the late 1920's of a technique somewhat similar to that disclosed in Troller to attempt to increase the efficiency of Kaplan turbines.
The foregoing indicates that various attempts have been made to increase the efficiency of air propellers and Kaplan turbines. However, none of these attempts have been utilized to increase the survivability of fish as they pass through these turbines. Moreover, in view of the fact that prior art systems and methods to divert fish away from Kaplan turbine are costly and not fully satisfactory, there is a need to find other ways to lessen the ability of Kaplan turbines to injure fish, thereby enhancing the environmental characteristics of these turbines, while increasing, or at least not impairing the overall efficiency of these installations.